Summary:
5-Keto-D-gluconate 5-reductase catalyzes the reversible reduction of 5-ketogluconate to D-gluconate. This is the second reaction of the L-idonate catabolic pathway after uptake of L-idonate into the cell. The enzyme specifically reduces 5-ketogluconate using either NADH or NADPH. D-gluconate oxidation by the enzyme can only proceed with NAD as the coenzyme; NADP only results in low specific activity [Bausch98].

Based on sequence similarity, IdnO was predicted to be an acetoin dehydrogenase (diacetyl reductase) [Reed03].

Expression of the idnDOTR operon is catabolite repressed and induced by L-idonate or 5-ketogluconate [Bausch04].

Summary:
The specific activity of the enzyme in the 5-keto-D-gluconate 5-reductase direction is more than 10-fold higher than in the oxidation direction [Bausch98]. Although the enzyme can utilize both NADH and NADPH, it has three times the activity using NADH [Yum99].